This invention relates generally to bearings and, more particularly, to bearings in which one or more magnetic field detectors provide information concerning the angular position of one element in relation to another.
At present, several detectors, such as inductive or variable reluctance pickup detectors, are used to detect signals indicating the position, speed, or acceleration of rotary elements. These detectors generally measure changes in the intensity of magnetic field as a ferromagnetic rotor or tooth passes in front of the detector. The frequency of the detector signal is used to calculate the revolutions per minute of the revolving element. These detectors have been added to bearings in designs intended to save space, increase measurement accuracy, and simplify installation.
U.S. Pat. No. 3,716,788 describes a variable reluctance detector added to a wheel bearing assembly to measure the speed of the axle. A toothed rotor is provided on a rotating inner bearing ring opposite a toothed stator, which is attached to the outer fixed ring of the bearing. The outer ring also holds the magnetic pickup, which generates a voltage as a function of changes in magnetic flux induced by the alignment of the teeth of the stator and rotor during rotation. The frequency of the alternating current produced is proportional to the rotational speed of the axle. A comparable speed detector is described in U.S. Pat. No. 3,826,933. That speed detector has a magnetic pickup coil attached to the fixed ring of a roller bearing, while a wheel with ferromagnetic teeth is attached to the rotating ring of the bearing.
U.S. Pat. No. 4,069,435 describes a device for detecting relative motion in a bearing. That device comprises a magnetic pickup coil mounted on the seal of the fixed outer ring of a bearing. A toothed wheel is mounted on the rotary ring of the bearing. When the teeth are rotating, variations in the magnetic field are detected by the magnetic pickup. The frequency of the variations detected can be used to calculate both speed and acceleration.
According to U.S. Pat. No. 4,688,951, an electrical circuit is connected to one of the two rings of a bearing to create the magnetic field.
The known detection devices described above have many disadvantages. Variable reluctance detectors generate an output voltage signal whose frequency and amplitude are proportional to the speed.
At low speeds, the output signal is weak and may be inaccurate. Moreover, when the aforementioned detectors are added to bearings, the bearings require more space because of the added detection components. The mechanical precision and accuracy of the signal decreases due to the accumulation of tolerances, and the structure of the assembly becomes more complex. Moreover, the bearings are difficult to install, and sometimes the installation process must be separate from that of the detector.
U.S. Pat. No. 4,875,785 describes a bearing with a magnetic field detector, in which the field is produced by an annular ferromagnetic ring mounted on a rotating shaft, which is permanently magnetized to form multiple segments.
During operation, the pickup delivers a position signal in relation to a reference position by counting up or counting down (depending on the direction of rotation) the number of quantities corresponding to incremental units.
When the bearing begins to operate, the absolute position of the shaft is known only when the rotor has passed the reference position.
The foregoing illustrates limitations known to exist in present bearings with angular position pickups. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.